The invention arose during continuing developmental efforts by the assignee of the present application in seeking improved vibration control for rotary machinery, namely rotary knives as disclosed in co-pending patent application Ser. No. 09/045,466 entitled "A Rotary Knife With Active Vibration Control" by James R. Michler, incorporated by reference herein. During the development of active vibration damping systems using electromagnetic actuators, it was found that the actuators contributed a substantial phase lag to the control system. It was further noticed that implementing such an active damping system without accounting for the phase lag caused a reduction in performance. Therefore, it was desirable to have the control system account for the phase lag caused by the actuator. However, the phase lag was so substantial that conventional techniques, such as phase lead controllers, rendered the system potentially unstable in practice.
The invention involves the use of a new technique for accommodating excessive phase shifts in an active vibration damping system without rendering the system unstable.